In the manufacture of fiber reinforced resin products, sheet-molding compounds are frequently used. Sheet molding compounds offer an appealing solution for the production of Class A surface parts compared to steel both in terms of cost and coefficients of thermal expansion.
Sheet molding compounds consist of a mixture of a thermosetting resin, a thermoplastic (typically dissolved in styrene) and catalyst, particulate filler and chopped reinforcement fibers, such as glass fibers. In most cases, the resin and chopped fibers are sandwiched between films of plastic material to form a laminated sheet that is wound in rolled form or festooned for storage. The laminated sheet is stored under conditions that will not result in final curing of the resin, but will allow the paste to thicken from typically a 10,000 to 40,000 centipoise (MilliPascal seconds) range to a desired molding viscosity range, typically between 30,000,000 and 50,000,000 centipoise. At the time of use, the protective carrier film is removed and the laminated sheet is cut into blanks, or plies, of a desired shape and size. The plies are then molded to form a cured composite part. In most applications, multiple plies of the laminated sheets are used in the composite structure and typically comprise between 25 and 50% of the die/tool's surface area. When the laminated sheets are molded, the resin and glass flow within the mold under heat and pressure to cover the entire surface of the mold. Sheet molding compounds are used in a variety of applications that require aesthetic appeal, corrosion resistance, lighter weight dimensional control and high strength.
One potential way to produce sheet molding compounds has been to sandwich a wet process textile mat, instead of chopped fibers, between layers of sheet molding compound resin and molding the resultant laminate into a composite part. However, the fiber contained within the wet process textile veil mat does not flow well under pressure. Thus, the composite parts formed by this process have similar poor surface characteristics as composite parts formed with chopped fibers.
It is therefore highly desirable to improve the characteristics of sheet molding compound. This would allow sheet molding compound parts to be used in a wider variety of composite applications wherein surface quality as well as scrap and rework is a concern.